Intelligent Life: Numbers For The Real World

In the movie "Contact," Jodie Foster says,
"There are 400 billion stars out there, just in our galaxy alone. If just
one out of a million of those had planets, and just one in a million of
those had life, and just one out of a million of those had intelligent life,
there would be literally millions of civilizations out there."

If there are 400 billion stars out there, and one out of a million of those
has planets, and one out of a million of those has life, and one out of a
million of those has intelligent life, then the chances of there being
intelligent life here on Earth are a million to one. We'd better start
looking.

Here's how the Mathemagician explains it:

If there are 400 billion stars out there and one out of a million of those
has planets, then 400 billion divided by 1 million have planets. A billion
divided by a million is a thousand (or a million in Britain, but it's an
American movie). Out of those 400 thousand planets, if one out of a million
has life, then 400,000 divided by 1 million (1,000,000) has life. That's
0.4 planets with life, or a 4 out of ten (or one out of 2.5) chance of
finding any planet with life at all. Now, if one out of a million of those
have intelligent life, then there's about a one in 2.5 million (continuing
to divide by a million) chance that there's intelligent life anywhere. The
unfortunate part is that the character who makes the statement is an
astronomer who has spent her life on this search!

Of course, she's only considering one galaxy. As my friend Dave Dyson (of the
Un-Scripted Theater Company) points out,
there are about 250 billion galaxies out there, and we now know that there is
intelligent life in every 2.5 million of those. That would mean about 100,000
planets with intelligent life. Still not millions, but if Jodie had considered
all galaxies, she wouldn't be that far off.

Dave also says "If there are other civilizations out there, what are the chances
they exist either a) currently, b) at the time in the past when light would have
left them on its way to us, so they could potentially be observable by us, or
c) at such time in the future when we can visit them?" So even considering other
galaxies, we have to limit ourselves to civilizations that exist close enough to
our own time.

Today's fact: You are more likely to be killed by a snake than to be killed
in an airplane crash. Animal Trivia at http://www.baringvet.net/trivia.htm
says that you have a 1 in 3,000,000 chance of being killed by a snake. You
only have about a 1 in 6,000,000 chance of being killed in a plane crash,
according to the cheerful people at http://www.amigoingdown.com.

The Mathemagician has also heard that you are more likely to be kicked to
death by a donkey than killed in an airplane crash, but some donkey lovers
have written to him claiming that it's just anti-donkey propaganda. This
quote also seems to be somewhat out-of-date. Here is confirmation from a
reader (thanks, Mike!):

"The original quotation (ex USAF flight safety briefing) goes:

'In the year 1947 in the United States of America more people were kicked to
death by donkeys than were killed in civilian flying accidents'

Trying to get the (London) American Embassy to confirm this from census
figures used to be an initiative test!"